What is the most likely cause and acute management for a unilateral facial weakness that spares the frontalis muscle?

Unilateral Facial Weakness with Frontalis Sparing: Central (Upper Motor Neuron) Lesion

When the frontalis muscle is spared in unilateral facial weakness, this indicates a central (upper motor neuron) lesion—most commonly an acute ischemic stroke—and requires immediate stroke protocol activation with urgent brain imaging and neurovascular assessment. 1

Anatomical Basis for Frontalis Sparing

• The frontalis muscle receives bilateral cortical innervation from both cerebral hemispheres, which is why upper motor neuron lesions typically spare forehead function 2, 1
• In contrast, peripheral facial nerve paralysis (Bell's palsy) affects the entire ipsilateral face including the forehead because the lesion is below the level of bilateral cortical input 2, 1
• This distinction is critical: forehead involvement = peripheral lesion; forehead sparing = central lesion 1

Most Likely Cause: Acute Ischemic Stroke

• Stroke is the primary diagnosis to consider when facial weakness spares the forehead, often presenting with sudden onset 1
• Additional neurologic deficits frequently accompany central facial palsy, including dizziness, dysphagia, diplopia, or limb weakness 2, 1
• Recent evidence challenges the traditional teaching: up to 76% of patients with acute ischemic stroke may demonstrate some upper facial weakness, though it is typically milder than lower facial involvement 3
• Greater stroke severity (higher NIHSS scores) and presence of lower facial weakness predict upper facial involvement 3

Acute Management Algorithm

Immediate Actions (Within Minutes)

• Activate stroke protocol immediately and obtain stat non-contrast head CT to exclude hemorrhage 1
• Assess for thrombolytic eligibility if presenting within 4.5 hours of symptom onset
• Perform focused neurologic examination looking for other cranial nerve deficits, limb weakness, sensory changes, ataxia, or dysarthria 2, 1
• Check vital signs and establish IV access

Diagnostic Workup

• MRI brain with diffusion-weighted imaging is the gold standard for detecting acute ischemic stroke and should be obtained urgently 2
• Vascular imaging (CT angiography or MR angiography) to assess for large vessel occlusion
• ECG and cardiac monitoring to evaluate for atrial fibrillation or other arrhythmias
• Do NOT delay imaging to obtain routine laboratory tests in the acute setting 4

Differential Diagnosis to Exclude

• Brain tumor or mass lesion (gradual onset over days to weeks, not acute) 1
• Brainstem infarction or hemorrhage (often with multiple cranial nerve involvement) 2
• Demyelinating disease such as multiple sclerosis (younger patients, relapsing-remitting course) 2, 5
• Rarely, isolated facial nerve palsy from cortical or brainstem infarct can occur without other deficits 2

Key Clinical Pitfalls to Avoid

• Never assume all acute facial weakness is Bell's palsy—30% of cases have identifiable causes requiring different management 1
• Always assess forehead function by asking the patient to raise eyebrows and wrinkle forehead; this single maneuver distinguishes peripheral from central causes 1, 4
• Do not overlook subtle additional neurologic signs that indicate central pathology 1
• Recent research shows upper facial weakness can occur in central lesions, so mild forehead involvement does not exclude stroke—look for asymmetry in severity between upper and lower face 3
• Bilateral facial weakness is extremely rare in Bell's palsy and should prompt investigation for Lyme disease, Guillain-Barré syndrome, or sarcoidosis 1

When Central Facial Palsy is NOT Stroke

If stroke is excluded by imaging and clinical course, consider:

• Neoplastic causes: brain tumor, skull base tumor, or parotid malignancy with perineural spread 2, 1
• Inflammatory conditions: sarcoidosis, multiple sclerosis, or other demyelinating disorders 2, 5
• Infectious etiologies: Lyme disease (especially if bilateral), neurosyphilis, or HIV-related complications 1, 5
• MRI head with and without IV contrast is the appropriate imaging modality for these alternative diagnoses 2